1. Field of the Invention
Typically, an automatic transmission for an automotive vehicle comprises a hydraulic torque converter and a multi-speed shift mechanism. The torque converter reduces or multiplies the engine output torque and transmits it to its turbine shaft. The multi-speed shift mechanism shifts the transmission into desired speeds or gears so as to further reduce and transmit the engine output torque to rear wheels of the vehicle. The multi-speed shift mechanism also reverses the rotation of the turbine shaft of the torque converter and transmits it to the rear wheels.
A multi-speed shift mechanism typically comprises a planetary gear set, various friction clutch elements for coupling and releasing the turbine shaft from various gears of the planetary gear set and various friction brake elements for braking specific gears of the planetary gear set so as to shift transmission gear ratios. The coupling or engagement of these friction clutch elements and friction brake elements is selectively controlled by regulating the line pressure in the automatic transmission by a hydraulic pressure control system.
2. Description of Related Art
Some automatic transmissions of this kind, such as the transmission described in Japanese Patent Publication No. 61-48021, have a line pressure control map of predetermined line pressures according to shift patterns and engine loads. However, since the engine load is basically unchanged during shifting, the line pressure is kept unchanged for the same shift pattern. Consequently, it is difficult to develop a line pressure suitable for an engine operating condition during shifting. This causes an abrupt coupling of the friction coupling elements, which generates large shift shocks in the automatic transmission, particularly during down-shifting.
Experience has shown that during proper down-shifting, the engine must be able to raise its speed rapidly and with high response at the beginning of down-shifting. Thereafter, the engine must change the coupling speed of the frictional elements of the automatic transmission by modifying the coupling force in accordance with the engine output torque. It is considered desirable for proper operation to gradually raise the line pressure as the turbine speed of the torque convertor increases. However, it is somewhat difficult, with an automatic transmission line pressure control system providing such a control, to provide a proper line pressure. This is because during down-shifting, for example, from a third speed gear to a second speed gear, caused by a manual change of transmission range while the accelerator pedal is released, a line pressure, suitably developed for the turbine speed, has a pressure value corresponding to the engine load which is higher than a line pressure developed when the accelerator pedal is released. For this reason, it is considered desirable to set the line pressure to an ordinary line pressure, developed when no shifting is present, without using a line pressure setting map, which is ordinarily used to determine a line pressure in the automatic transmission according to turbine speeds, only when down-shifting is caused by a manual change of the transmission range. However, because the normal line pressure is set to a pressure suitable for transmitting the engine brake torque at a turbine speed after shifting, even when the engine is subjected to no engine load or the accelerator pedal is fully released, the line pressure does not correspond to a turbine speed during shifting and, therefore, is set relatively high. Accordingly, in this case, it is difficult to properly develop the line pressure.
Furthermore, in the automatic transmission, during up-shifting, for instance, from a second speed gear to a third speed gear, the line pressure in the automatic transmission is varied by the hydraulic pressure control system so as, on one hand, to release a 2-4 brake element and, on the other hand, to couple or engage a coast clutch element. As is well known, the 2-4 brake element typically comprises a brake band equipped with a servo piston, and is tightened when a hydraulic pressure is supplied to an apply port of the 2-4 brake element only and is released when a hydraulic pressure is supplied both to the apply port and a release port of the 2-4 brake element. Since the 2-4 brake element is under a hydraulic pressure at the apply port while the automatic transmission is placed in the second speed ratio, it is necessary, during up-shifting from the second gear ratio to the third gear ratio, to apply a hydraulic pressure to the release port of the 2-4 brake element so as to release the 2-4 brake element and, simultaneously, to apply a hydraulic pressure to the coast clutch element so as to engage or couple the coast clutch element. For this reason, pressure lines connected to the release port of the 2-4 brake element and the coast clutch element, respectively, are connected to an output port of a shift control valve.